Field of the Invention
The invention relates to a clamping circuit that has cross-coupled first and second transistors for generating a predetermined minimum voltage. The clamping circuit changes over from a normal mode to a clamping mode if a voltage of a signal fed via an input path falls below a predetermined clamping voltage.
Clamping circuits are generally used to keep the level of an applied signal at a specific minimum value. Such clamping circuits have acquired considerable importance in the application of integrated circuits. This is because if input signals drop into a region of a diode voltage that is below ground potential or less, it could cause currents to flow via the parasitic components that are present in every integrated circuit. These currents can interfere with neighboring components or even the entire function of the circuit. The risk is particularly high for example when a fault in the form of an interruption of a ground connection occurs in an electronic circuit configuration having a plurality of supply voltages and ground connections. Particularly in safety-critical applications (for example in electronic systems appertaining to automobile electronics), it must be ensured that the circuit sections which are not directly affected by the fault are not influenced.
In order to solve this problem, a circuit having four npn transistors and a current source, is known, for example. The first and second transistors, which each have a very steep output characteristic curve, are cross-coupled.
The emitter of the first transistor (output transistor) is connected to an input path on which the input signal to be monitored is present. Such a circuit makes it possible to obtain a good protective effect in the clamping mode, the desired clamping voltage being adhered to in a very accurate manner. However, the known circuit has the disadvantage that it is not suitable for operation with high input voltages (for example 40 V or more). This is due to the fact that the first transistor, which is an NPN transistor, has only a low capacity to withstand such positive input voltages on account of its relatively low emitter-base breakdown voltage.
To remedy this, it is known to make the circuit voltage-proof with a first diode between the emitter of the first transistor and the input path for the input voltage, and a second diode at the emitter of the second transistor being necessary for reasons of symmetry. As a result, the desired clamping voltage can be adhered to. However, this circuit has the disadvantage that the current-dependent forward voltage of the first diode distorts the clamping voltage and the protective effect is thus greatly impaired in the clamping mode. Although this problem can be partly resolved by increasing the current I.sub.bias, the consequence of this measure is that the current through the first transistor actually increases before the predetermined clamping voltage is reached, and, as a result, the total current consumption of the clamping circuit in the normal mode also increases in an undesirable manner.
U.S. Pat. No. 5,519,341 discloses a comparator circuit with cross-coupled transistors, which detects the load current through a transistor by a source resistor and detects it to a predetermined current value. The current through the transistor can be limited by a flip-flop.
U.S. Pat. No. 5,576,616 likewise discloses an SA with cross-coupled transistors, which serves as a reference voltage source for an integrated circuit configuration, in which the supply potential may fluctuate. The circuit configuration specified is, moreover, insensitive to temperature fluctuations.
Published, Non-Prosecuted German Patent Application DE 25 49 575 A1 describes a circuit configuration with cross-coupled transistors, which is provided for connection to a specific current or voltage source. This circuit generates a signal that is independent of the current or voltage source.